Patent EA-002008-B1 (EA)
Title: Processof liquifying a gaseous, methane-rich feed to obtain liquefied natural gas
Inventor: HODGES DEREK WILLIAM
IPC: F25J1/00
Filed: 1998-12-11
Abstract: 1. Process of liquefying a gaseous, methane-rich feed to obtain a liquefied product, which liquefaction process comprises the steps of: (a) supplying the gaseous, methane-rich feed at elevated pressure to a first tube side of a main heat exchanger at its warm end, cooling, liquefying and sub-cooling the gaseous, methane-rich feed against evaporating refrigerant to get a liquefied stream, removing the liquefied stream from the main heat exchanger at its cold end and passing the liquefied stream to storage as liquefied product; (b) removing evaporated refrigerant from the shell side of the main heat exchanger at its warm end; (c) compressing in at least one refrigerant compressor the evaporated refrigerant to get high-pressure refrigerant; (d) partly condensing the high-pressure refrigerant and separating the partly-condensed refrigerant into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction; (e) sub-cooling the heavy refrigerant fraction in a second tube side of the main heat exchanger to get a sub-cooled heavy refrigerant stream, introducing the heavy refrigerant stream at reduced pressure into the shell side of the main heat exchanger at its mid-point, and allowing the heavy refrigerant stream to evaporate in the shell side; and (f) cooling, liquefying and sub-cooling at least part of the light refrigerant fraction in a third tube side of the main heat exchanger to get a sub-cooled light refrigerant stream, introducing the light refrigerant stream at reduced pressure into the shell side of the main heat exchanger at its cold end, and allowing the light refrigerant stream to evaporate in the shell side, (e) controlling the liquefaction process using an advanced process controller based on model predictive control to determine simultaneously control actions for a set of manipulated variables in order to optimize at least one of a set of parameters whilst controlling at least one of a set of controlled variables, wherein the set of manipulated variables includes the mass flow rate of the heavy refrigerant fraction, the mass flow rate of the light refrigerant fraction and the mass flow rate of the methane-rich feed, wherein the set of controlled variables includes the temperature difference at the warm end of the main heat exchanger and the temperature difference at the mid-point of the main heat exchanger, and wherein the set of parameters to be optimized includes the production of liquefied product. 2. Process according to claim 1, characterized in that the set of controlled variables further includes the temperature of the liquefied stream removed from the main heat exchanger. 3. Process according to claim 1 or 2, characterized in that the set of manipulated variables further includes the speed of the refrigerant compressor(s) in order to maximize the utilization of the compressors. 4. Process according to any of claims 1 to 3, wherein partly condensing the high-pressure refrigerant in step (d) is done in at least one heat exchanger by means of indirect heat exchange with propane evaporating at a suitable pressure. 5. Process according to any of claims 1 to 4, wherein the gaseous, methane-rich feed is obtained from a natural gas feed by partly condensing the natural gas feed to obtain a partly condensed feed. 6. Process according to claim 5, wherein partly condensing the natural gas feed is done in at least one heat exchanger by means of indirect heat exchange with propane evaporating at a suitable pressure. 7. Process according to claim 5, further comprising fractionating the partly condensed feed in a scrub column to get a gaseous overhead stream and a liquid, methane-depleted bottom stream; and partly condensing the gaseous overhead stream and separating the gaseous overhead stream into a gaseous, methane-rich stream which forms the gaseous, methane-rich feed and a liquid bottom stream of which at least part is passed to the scrub column as reflux, characterized in that the set of manipulated variables further includes the temperature of the liquid, methane-depleted bottom stream, in that the set of controlled variables further includes the concentration of heavier hydrocarbons in the gaseous, methane-rich stream, the concentration of methane in the liquid, methane-depleted bottom stream, the mass flow rate of the liquid, methane-depleted bottom stream and the reflux mass flow rate, and in that the set of parameters to be optimized further includes the heating value of the liquefied product. 8. Process according to claim 7, further comprising adding a butane-containing stream to the reflux, characterized in that the set of manipulated variables further includes the mass flow rate of the excess liquid bottom stream and/or the mass flow rate of the butane-containing stream. 9. Process according to any of claims 7 or 8, wherein partly condensing the gaseous overhead stream is done in at least one heat exchanger by means of indirect heat exchange with propane evaporating at a suitable pressure. 10. Process according to any of claims 4, 6 or 9, wherein evaporated propane is compressed in at least one propane compressor stage and condensed by heat exchange with an external coolant, characterized in that the set of manipulated variables further includes the speed of the propane compressor(s), and in that the set of controlled variables further includes the suction pressure of the first propane compressor. 11. Process according to any of claims 1 to 10, further comprising reducing the pressure of the liquefied stream to get the liquefied product which is passed to storage and an off-gas; and compressing in an end-flash compressor the off-gas to get high-pressure fuel gas, characterized in that the set of controlled variables further includes the loading of the end flash compressor. 12. Process according to any of claims 10 to 11, further comprising separately controlling the bulk composition and the bulk inventory of the refrigerant.